ABSTRACT: An algorithm is developed to compute the conductivity of lake and dilute ocean water from measured chemical composition at arbitrary temperature and pressure. The complex mixed electrolyte is considered as a sum of binary electrolytes rather than a sum of ions. Effects of ion association are included, and it is found that pairing effects are important in natural freshwaters. Bounds on the accuracy of the algorithm for specific classes of binary electrolytes are assessed and it is estimated that the algorithm has an overall accuracy of better than 2% for salinities less than about 4 g L1. Comparison with seawater conductivities is much better than 1%, but predicted conductivities of some published analyses of river waters are about 3% too high. Some of this difference may be due to a lack of data on ion pairing effects between bivalent metals and bicarbonate, but also may result from uncertainties in the measured chemical composition and measured conductivity. An iterative procedure incorporating this algorithm is used to compute reference conductivity at 25°C and salinity from in situ measurements of conductivity in waters where only relative amounts of ions are known. It is found that the conversion to reference conductivity is reasonably independent (to within about 1%) of the ionic composition for most world river waters, but is somewhat different than that for KCl solutions. However, derived salinities are quite sensitive to the composition, and the ratio of ionic salinity to reference conductivity varies between 0.6 and 0.9 mg L1 (µS cm1)1.